Steel cords have been used variously for the purpose of reinforcing composites by being embedded in matrices, e.g., rubber. Most of all, the composite of rubber and steel cord in which the rubber is reinforced by the steel cord has been widely applied to various rubber products, e.g., tires, belts, and hoses, because the rubber products can be provided with the strength and rigidity, which are insufficient when the rubber is used alone.
A product including such a composite is produced usually through a step of molding while a matrix is in a fluidized or softened state. In such a process, the steel cord used for reinforcement is stiff and, therefore, the flexibility in molding is frequently restricted. Consequently, regarding the steel cord in the related art, an enhancement of the strength and the rigidity of the product and the flexibility in a production process are mutually contradictory.
In particular, tire is in the shape of a circle, and curved surfaces make up the major part thereof. Therefore, flexibility is required in the production process. Particularly in the vulcanization step thereof, expansion is usually conducted in an oven in order to fit to a mold. On the other hand, regarding a completed product, high strength and rigidity and dimensional stability are important to endure long-term severe use and deliver stable performance. In particular, a crown portion of the tire is constantly subjected to a tensile force in a circumferential direction because of an internal pressure during use, and the circumference is elongated through creep due to use. Consequently, strain may be generated so as to deteriorate the durability and a cross-sectional shape of the tire may be changed so as to deteriorate the wear resistance characteristic.
On the other hand, as for a technology related to strengthening a tire crown portion, for example, Patent Document 1 discloses a technology of efficiently preventing separation without increasing the weight of the tire by disposing two interlacing layers and at least one crown strengthening layer located under the interlacing layers and formed from a strip, in which a reinforcing element composed of a plurality of corrugated or zigzag cords (or filaments) are oriented along an equator as a whole, in a tread portion around a carcass.
Furthermore, in Patent Document 1 described above, it is also disclosed that the use of the strip serving as the crown strengthening layer, in which a plurality of corrugated or zigzag cords or filaments are oriented along an equator as a whole, is simple in the production because elongation is achieved easily in vulcanization.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2-208101 (Claims and the like)